Small Engines (Lawn Mowers, etc.): Backfires after replacing counterweight link., briggs and stratton, valve clearances

Question
My B&S 18.5 hp (31p777-0296-e1) lawn tractor broke one of the counterweight link (arms).  I stripped it down and replaced both links.  After reassembling, the engine now fires a few times after a bit of cranking but then backfires and stops.  When I reassembled the unit I aligned the mark on the crankshaft with the mark on the camshaft.  They flywheel key is fine.  I had difficulty determining which valve was the intake and exhaust to put the proper push rod back in (steel vs aluminum).  From what I could read it said the intake was usually the top valve so that's how I placed them. I don't believe the valve clearances changed changed during the reassembly.  Where should I start?

Answer
Re-check the flywheel key.  Need to check the valve gap.  See this link for the specs:

http://www4.briggsandstratton.com/miscpdfs/RNT/Engine%20Specifications%20Chart_m...

Also, Briggs and Stratton "Customer Service" has the valve adjustment procdures.

Eric